(725g) Effect of Confinement On the Phase Behavior of Wetting and Nonwetting Adsorbates: Combining High Resolution Gas Adsorption and Mercury Porosimetry for An Advanced Textural Characterization of Mesoporous Molecular Sieves

In order to investigate the effects of confinement on the phase behavior of nonwetting fluid (Hg)  and wetting fluids (Nitrogen and argon at their respective boiling temperatures) we performed a systematic study of the sorption behavior of N₂ at 77 K and Hg porosimetry at 298 K in both 3-D and 2-D ordered large pore systems, i.e., cubic Ia3d KIT-6 and hexagonal p6mmSBA-15 silica materials with pore diameters ranging from ca. 7 nm up to 11 nm

Sructural details of  ordered mesoporous silica, i.e. KIT-6 and SBA-15 were obtained by performing a systematic study of nitrogen adsorption at 77 K including so-called scanning experiments [1] of the adsorption hysteresis loops (which allows, for instance to obtain details about pore network characteristics). Applying proper density functional theory methods allowed us to obtain accurate pore size data of a series of KIT-6 and SBA-15 silica materials with tailored mean pore diameters. The effect of pore network characteristics on mechanical stability were probed by performing careful mercury intrusion/extrusion experiments up to pressures of ca. 4000 atm. No collapse of the pore structure was observed when Hg intrusion/extrusion cycles were performed on fully 3-D mesopore systems, i.e.KIT-6, even for materials with pore diameters up to 13 nm.

To the best of our knowledge this is the first successful example of Hg porosimetry on ordered mesoporous molecular sieves such as KIT-6 silica. Depending on details of the pore networks characteristics reversible mercury intrusion/extrusion cycles or collapse of the pore network was found for SBA-15 silica samples.

Our results using these ordered mesopore structures confirmed that the underlying mechanism of mercury intrusion/extrusion is thermodynamically equivalent to the mechanism of capillary evaporation/condensation of a wetting fluid. From the analysis of the mercury intrusion/extrusion data we derive mercury vapor adsorption/desorption  isotherms which demonstrate the effect of pore diameter on the vapor-liquid phase transition which occurs for the non-wetting fluid mercury at pressures which are larger than the saturation vapor pressure p₀of the bulk fluid, the smaller the pore diameter the larger the relative pressure where the condensation of mercury occurs.    

Combing advanced physical adsorption and mercury porosimetry results provides new insights into the correlation between the pore structure/pore network characteristics and the properties (including mechanical stability) of ordered mesoporous silica materials.

 [1] R.Cimino, K.A Cychosz, M. Thommes, Neimark, Colloids and Surfaces A, in press (2013)

[2] a) F. Porcheron, P.A. Monson, M. Thommes, Langmuir, 2004, 20, 6482; b) F. Porcheron, M. Thommes, R. Ahmad, P.A. Monson, Langmuir, 2007, 23, 3372; c) M. Thommes, R. Skudas, K.K. Unger, D. Lubda, J. Chromatogr A, 2008, 1191, 57.